This invention relates to thin-walled structures (such as extruded honeycombs) of activated carbon, to a method for their preparation, and to their use for the passive adsorption of hydrocarbon vapors. More particularly, the invention relates to such structures that can be made through a method that does not require firing or sintering at elevated temperatures, and to the use of the honeycomb structures as a pollution control device to prevent evaporative emission of hydrocarbons to the atmosphere from, for example, motor vehicle fuel tanks and gasoline pump nozzles.
As part of the pollution control devices now used in automobiles, each automobile is equipped with a canister of activated carbon for the purpose of adsorbing hydrocarbon vapors that are emitted from the fuel tank or from the carburetor by natural evaporation. The hydrocarbon vapors generated while the engine is idle are vented from the fuel tank and carburetor to the canister to be adsorbed on the activated charcoal and thereby charcoal is later purged during engine operation by drawing air through the charcoal via the air intake to the engine.
Although this system is generally effective, it does have drawbacks that are based primarily on the geometric arrangement of the activated charcoal in the canister. As used today, the activated charcoal resides in the canister in the nature of a bed of granulated material. The adsorption capacity of a packed bed, however, is substantially less than theoretical capacity because gas diffusion through a packed bed is not uniform and the gas actually comes into contact with only a minimal portion of the available surface area. Therefore, the practically (as opposed to theoretically) available charcoal becomes saturated and additional hydrocarbon vapors generated pass the charcoal without being adsorbed and are left to be emitted to the atmosphere. Although this problem could be overcome by use of a greater volume of charcoal, the cost and space constraints make such a solution impractical.
An alternative to such a system is the use of a thin-walled honeycomb structure of activated carbon having a plurality of through-and-through channels, which has the advantage of more efficient packing of material per unit volume, providing greater practical surface area for adsorption, than is provided by a bed of granulated material. The use of such a honeycomb for the passive adsorption of evaporative emissions from a fuel tank is shown in U.S. Pat. No. 4,386,947, which discloses a monolithic honeycomb of activated carbon and combinations of activated carbon, a fuel-resistant resin and porcelain for adsorbing evaporative fuel emissions in an automotive anti-pollution system. The activated carbon honeycomb is prepared by making a plasticized batch of the activated charcoal, a water-soluble varnish resin containing a water-soluble imide resin, water, and methylcellulose; extruding a green honeycomb; exposing the honeycomb to progressively higher temperatures from 60.degree. C. through 120.degree. C., followed by heating at 270.degree. C. in a nitrogen atmosphere.
Other methods for preparing activated-carbon honeycombs are also known in the art. For example, Hucke, U.S. Pat. No. 3,859,421, discloses methods of producing carbonaceous bodies by casting a mixture of carbon-yielding binders such as furfuryl alcohol and furfuryl alcohol resins, polymerizing the alcohol by exposure to mineral acids or organic acids, consolidating the mixture by removal of liquids, and pyrolyzing at elevated temperatures to generate carbon.
Sugino, U.S. Pat. No. 4,399,052, discloses the extrusion of a mixture that contains at least one thermosetting organic resin into a honeycomb structure. The structure is then dried to eliminate hydrocarbons and effect carbonization, followed by heating to 600.degree.-1000.degree. C. in an oxidizing atmosphere to form an activated carbonaceous honeycomb body.
Honeycomb structures which can be prepared in a manner that eliminates the need for firing yet which nonetheless have the strength to withstand the vibrations and other stresses attending automotive use are now provided by the present invention.